If Chlorine is bonding with sodium it has no covalent bonds. The bond formed is an ionic bond. This is true of the bonds Chlorine forms with most positive elements.
If Chlorine is bonding with Oxygen it can form a single covalent bond
or one double bond and one single covalent bond
or two double bonds and one single covalent bond
or three double bonds and one single covalent bond
Because Chlorine has 7 valance electrons it can lose from 1 to seven to form bonds with Oxygen and other elements with greater electron negativity, creating up to 3 double bonds. Chlorine can also gain electrons from elements with a lower electron negativity.